Traditionally, aqueous-based polymer dispersions have been prepared via well known techniques such as suspension polymerization, emulsion polymerization, charge neutralization and high impact agitation, such as microfluidization. However, many of these techniques suffer from drawbacks, such as the need for stabilizers which can detract or minimize the effectiveness of the final product, large particle size, high viscosity or poor long term stability.
There has been much effort expended to develop aqueous polymeric dispersions which are comparable in performance to solvent-based polymeric products. When designing such aqueous polymer systems, one factor to consider is the fact that different polymers exhibit different degrees of performance and exhibit specific types of properties. Hence ideally, a combination of polymers may be preferred to achieve the required properties for any particular application.
Two-component, aqueous-based polymer dispersions have been developed in an effort to reduce the amount of volatile organic solvents, which solvents are both economically and environmentally undesirable. However, it has not been possible to obtain products utilizing such polymer dispersions which possess all or most of the properties obtained from solvent-based products, such as bond and tensile strength, chemical and water resistance, heat resistance, gloss and abrasion resistance.
One approach to overcoming some of the above mentioned deficiencies is to blend various polymers such that the properties required for a final application are achieved. Blending of polymers is a well known technique. However, polymer blend ing is subject to many limitations such as chemical thermodynamic and kinetic limitations, and morphology problems leading to phase separation, high viscosity, and inhomogeneous dispersions or gelation/coagulation. The phenomena becomes more difficult when either more than two types of polymers are blended or when the molecular weight of the polymers is too high.
Blending of polymers in the form of homogeneous aqueous dispersions requires a sizeable amount of surfactant-based stabilizers a nd emulsifiers. These surfactants are normally very hydrophilic in nature and tend to remain in the final product. One adverse effect caused by the presence of the surfactant in the final product is a reduction in the bond strength and resistance properties. Additionally, surfactants are low molecular weight species which are easily extracted from polymers. This is detrimental particularly in certain end-use applications such as in direct or indirect food applications. Surfactants also are surface active agents and therefore tend to accumulate at substrate interfaces, thereby limiting the ultimate adhesive performance. Another drawback is the large particle size of the dispersed phase that results in poor film clarity and poor long term stability.
For one t o both manufacture and blend polymers involves preparing the polymers separately and then blending them together, in which case the process is separated into many parts and becomes very uneconomical and cumbersome. Blending of only commercially available polymers will result in a limited choice of materials, as well as little or no control over the properties of the polymers. In addition, the performance required of such polymeric substances when used on various substrates in end-uses such as adhesives, binders, coatings, sizes, saturants and sealants, especially with the advent of new types of difficult-to-adhere substrates and stringent application conditions.
It is well recognized that waterborne urethane/vinyl emulsions can be prepared for use in aqueous compositions such as those noted above, with the advantage over earlier systems of being essentially free of volatile organic compounds. Methods of obtaining similar urethane-acrylic copolymers are also known where chain-termination of the polyurethane prepolymer is conducted with a C.sub.1 -C.sub.4 alkyl alcohol to obtain only urethane linkages (i.e., urea-free linkages) in the final dispersion, thus leading to clearer films or coatings. However, these systems by themselves have some limitations. For example, in the field of flexible laminating adhesives, the adhesion properties and barrier to chemicals and solvents of the aqueous compositions typically are inferior to solvent-based systems.
Free-radical polymerization of vinyl monomers in the presence of a seed polymer is a well known emulsion polymerization technique. Here, the seed polymer can be of any nature, but the seed polymer is prestabilized with surfactants. The final product also contains a substantial amount of surfactant to maintain the stability of the product, which in turn adversely affects the final performance.
The incorporation of a phenol/formaldehyde condensation resin or an epoxy resin during the preparation of an aqueous polyurethane dispersion is also known. However, here the polyurethane prepolymer is prepared in a water-miscible low boiling solvent. Again, the presence of a solvent is detrimental both economically and environmentally. Typically, additional processing is required to strip away the solvent.
There is a growing need to develop aqueous polyurethane dispersions which have certain improved properties over dispersions thus far known in the art. As noted above, one of the major problems of combining polymers is their incompatibility or inhomogeneity. It would be desirable to provide an aqueous polymeric dispersion and methods for making same which will not be subject to one or more of the disadvantages of the prior art noted above.
This invention overcomes the problems of blending more than two polymers. At the same time, the methods for preparing the polymer dispersions allows for designing of a polymeric system for specific end-use applications. The polymer dispersions according to the present invention provide comparable or better performance than that of a solvent-based system. The present invention also provides several unexpected improvements in properties attributable to the choice of the polymeric performance enhancers being incorporated into a polyurethane/vinyl polymer.